Preparation of powders suitable for the manufacture of permanent magnets



United Sttes Patent O PREPARATION ,OF POWDERS SUITABLE FOR THE OF PERMANENT MAGNETS Nisg ,(lqnrad Tem n o nglan assigflor to one M is .Ftans a reitera i n of Pr s NoDrawing. Application May 15, 1953,

, .Serial No. ;3;55,4tli) priority, application great Britain May 22, 1952 9 .Claims. (Cl. 252-:-62.5)

Anonyme dite: Societe dElectro-Chirnie, dElectro e .et des Acierles Ele'ctriques dUgine, Paris,

lhis invention relates to the production of magnetisable powder, consisting of metallic iron, suitable for use in the ntanufacture of permanent magnets by the known method of compacting the'powder and magnetising the compacts. GFhe'invention also relates to magnets manufactured from such powder.

It is known that the coercive force of permanent mag- It is, there- In one known method of manufacturing iron powder for permanent magnets, certain oxygenated compounds of'iro'n such as'iion'for'mate or iron oxalate are decomposed' and reducedto metallic iron. It has been proposed, in' connec tion with this method, to introduce one or more compounds ofmetals such as calcium, magnesium, alumiiium and cadmium into the iron compound by "a' process of crystalising a mixed solution of such a compound 'andiof the iron compound prior to the decomposition and reduction steps in the process.

It is an 'object' of the present invention to provide a method of manufacturing magnetisa-ble powder consist- 'ing mainly of metallic iron, and of uniformly small crystal size, using as the starting material an iron oxide or hydrated iron oxide or iron hydroxide.

According Y to the invention, a method of manufactursteps of heating an initial mixture of an iron oxide, or a hydrated iron oxide or iron hydroxide or other iron cpmpourldwhich yields an iron oxide on heating, with a suitable alkaline earth metal compound and an alkali metal oxide, hydroxide or carbonate, the proportion'of alkalineearth metal compound used being such that" the atomic ratio of iron to alkaline earth metal in the mixture'is not less than 9:1, and the heating being carried out at such a temperature that reaction takes place between the iron compound and the alkali metal cornpou ndtoffo'rm alkali metal ferrite, treating 'the prodnet of the heating with water, removing the aqueous solution so formed, and heating the solid residue at a temperature below 500 C. in a'reducing atmosphere until "stands present in the residue is mainly reduced'to the mantis state. a

'WhenIthe crystals of which the iron powder is compo sedare-of'anisotropic shape, the greatest dimension of the crystal may be as much as 5,000 A asspecified.

abovejf-I-Iowever, inthe 'case of crystals inwhich the dimensions are all approximately equa'h all the dimensions are preferably not much greater than 1,000 A.' z

2,783,207 Patented Feb. .26, 1957 ICC red material for use in the method of the invention is alpha-ferric oxide of small crystal size and low density in the uncompressed state, for example, the materials available commercially under the name of Turkey red oxides.

The alkaline earth metals referred to herein are cal cium, strontium and barium, calcium being preferred, and the preferred alkaline earth metal compounds for use in the method of the invention are the oxides, hydroxides and carbonates, although other oxygen-containing compounds such as the'formates and oxalate's, or any other compounds convertible into alkaline earth metal oxides under the conditions of the reaction, are also suitable. The proportion of alkaline earth metal compound introduced into the initial mixture is preferably such that the final product contains a proportion of alkaline earth metal, probably in the form of an oxide, within the range of 0.1% to 10% by weight of the powder.

The preferred alkali metal compounds for use in the method of the invention are the'hydroxides and carbonates'of sodium "and potassium; the choice between the use of an hydroxide and of a carbonate depends upon the manner rev/nth the method of the invention is carr-ied out, as'will be made clear below. The proportion of alkali metal compound is not critical providedthat it suificient to effect the required reaction during the heating of the initial mixture: it is believed that reaction of the alkali metal compound -with at least a part of the iron compound present to'form an alkali metal ferrite, during the initial heating, is an essential feature of the method, as will be discussed more fully hereinafter. Accordingly it is desirable that sufficient alkali metal compound should be present to react with all the iron compound present.

In one method of manufacturing powder consisting mainly of iron, in accordance with the invention, a mixture of finely divided iron oxide or other iron compound yielding an iron oxide on heating, an oxygen-containing alkaline earth metal compoundas aforesaid, and an alkali metal hydroxide is heated at a temperature in the range of 500 C. to 800 C., the mixture being kept in a semi-liquid state during the heating due to fusion of the alkali metal hydroxide, and the reaction product is allowed .to cool and is then treated with water, the resulting aqueous solution is filtered oil and the residual solid is reduced by heating in hydrogen at a temperature between 250 C. and 500 C. 'The amount of alkali metal hydroxide used in this method should be suifi cient to ensure that the mixture is kept just fluid throughout the alkali fusion process. i

In another'method of carrying out the invention, a mixture of finely divided iron oxide or otheriron compound yielding'an iron oxide on heating, an oxygen containing alkaline earth metal compound as aforesaid,

and an alkali metal carbonate is heated at a temperature in the range of 700- C. to 800 C. to produce a sintered mass whichis then treated with water and reduced in a,

' 3 specified above. The alkali metal compound should be completely removed by the water treatment, but there may be a slight residual trace of alkali metal compound, which is immaterial.

The powder referred to in the preceding paragraph, when compressed and magnetised, forms permanent magnets which have a coercive force considerably higher than that possessed by magnets made from iron powder obtained from the same starting material by a similar method, but without the addition of an alkaline earth metal compound to the initial mixture of iron oxide, or other iron compound, and alkali metal compound. Calcium has been found to be more effective than either barium or strontium in this respect.

The initial heating step, during which fusion or sintering accompanied by ferrite formation take place, may be carried out in air, but preferably an eifcctively reducing atmosphere is present, for example, this step may be carried out in an atmosphere of oxygen-free nitrogen. The use of such an atmosphere results in greater consistency in the quality of the product, and usually also in an improvement in the coercive force of the resulting magnets.

The reduction of the solid residue remaining after the water treatment is effected by heating in a stream of hydrogen, preferably flowing at a rate of at least 300 litres per hour for every to grams of solid residue, and it has been found that the magnetic properties of the magnets obtained depend to some extent on the rate of flow of hydrogen and the duration of the reduction of the powder. As will be shown in the examples to be given hereinafter, both the coercive force and the remanence of the magnets are higher when the powder has been reduced for 2 hours in a stream of hydrogen flowing at a rate of 600 litres per hour for every 10 to 15 grams of solid residue, than in cases where the reduction has been carried out for 4 hours at a rate of flow of 300 litres per hour.

In a modification of the method of the invention, the initial mixture may include a minor proportion of one or more compounds, capable of decomposition and/or reduction to form the metal, of ferro-magnetic metals other than iron, for example, oxides, hydrated oxides, ferrites, or carbonates, of nickel and/or cobalt: in this case the powder obtained as the final product may consist essentially of an alloy or mixture of iron and the other ferro-magnetic metal or metals, with iron as the predominant constituent.

Some specific methods in accordance with the invention for producing iron powder, and a method of manufacturing permanent magnets from the powder thus obtained, will now be described by way of example.

Example 1 The iron oxide employed in the method of this ex- 7 ample is alpha-ferric oxide, FezOa, of the Turkey red type which is of small crystal size and low density owing to its method of manufacture, in which it is not subiected to a high temperature, and is therefore in a highly reactive state.

An intimate mixture of1,520 grams of alpha-ferric oxide of the type referred to above and 74 grams of powdered calcium hydroxide Ca(OH)z, is prepared, and this mixture is heated for '15. minutes at 750' C. with sutiicient potassium hydroxide to keep the mass in a scmi-liquid state, due to fusion of the potassium hydroxide, throughout the heating process. The product of the heating is allowed to cool and is treated with water, the residual solid material is separated by filtration from the aqueous solution resulting from the water treatment, and is washed thoroughly with boiling water and then dried. The dried material is crushed into particles of about 1 to 3 mm. diameter, and this coarse powder is heated for 4 hours at a temperature of 350 C. in a stream of dry and deoxynated hydrogen which is arranged to pass through the powder in such a way as to ensure intimate contact between the hydrogen and the powder, at a rate of about 300 litres per hour per 15 grams of powder.

The powder produced by this method consists mainly of metallic iron and contains a small amount of iron oxide and 3.6% of its weight of calcium substantially uniformly distributed throughout the iron powder, probably in the form of a calcium oxide-ferrous oxide solid solution. The powder is collected in a vessel in which the atmosphere consists of dried and deoxynated hydrogen, and to protect the powder from atmospheric oxidation during subsequent processing it is covered with a solution of phenol-formaldehyde resin in acetone, and the acetone is evaporated, so that the dried powder contains about 5% of its weight of the resin in the formof a coating on the individual particles of powder.

For the manufacture of a permanent magnet the resin coated iron powder prepared as described above is compressed in a metal die under a pressure of about tons per square inch. A specific compact produced in this manner had a density of 3.92 grams per cc. and, when magnetised by the application of a magnetic field of 10,000 oersteds, was found to have a coercive force of 530 oersteds and a remanence of 3,200 gauss.

By way of comparison with the magnet manufactured as described above, another magnet made in the same way from iron powder prepared from the same oxide and by the same method as described in this example, with the exception that the calcium hydroxide was omitted from the initial reaction mixture, has a density of 3.88 grams per cc., a coercive force of 330 oersteds, and a remanence of 2,400 gauss.

Example 2 An intimate mixture of 1,520 grams of alpha-ferric oxide of the type employed in Example 1, 74 grams of powdered calcium hydroxide and 1,311 grams of potas sium carbonate, is prepared by milling the mixed powders together for 50 hours under methylated spirit. The mixture thus prepared is dried and is sintered at a temperature of 800 C. for 1 hour. The sintered product is cooled, treated with water, filtered, crushed, and reduced in the manner described for the treatment of the prod net of the initial heating step in Example 1.

The powder thus obtained is coated with phenolformaldehyde resin, and compacted and magnetised, as described in Example 1, and a specific magnet thus produced was found to have a density of 3.8 grams per cc., :1 coercive force of 500 oersteds, and a remanence of 2,200 gauss.

Example 3 The iron powder is manufactured by a method which is a modification of the method of Example 1 in that the initial heating step is carried out in an atmosphere of oxygen-free nitrogen, at a temperature of 800 C., the heating being continued for 1 hour. The reduction step is carried out in the manner described in Example 1, and the subsequent treatment of the powder and the manu facture of magnets from the powder obtained are also carried out as described in Example 1.

A specific magnet produced in this manner Was found to have a density of 3.6 grams per cc., a coercive force of 590 oersteds and a remanenee of 2,000 gauss.

Example 4 The iron powder was manufactured by the same methed as that described in Example 3, with the exception that the reduction was effected by heating the powder at 350 C. for 2 hours in a stream of hydrogen flowing at a rate of 600 litres per hour per 15 grams of powder.

A specific magnet manufactured from this powder, by the method described in Example 1, was found to have a density of 3.6 grams per cc., a coercive force of 620 tes-aw ease-as and a remainehee ef 2,7 60 gsuss. By wayet comparison, anetner ma netmade in the same wyriem cc'.,. a coercive force of 360 oer'steds and a remanence of In the method of manufacturing magnetisable iron powder in accordance with the invention, the alkaline earth metal compound appears to perform the function of limiting crystal growth of the iron particles during the reduction step. Substantially uniform distribution of the alkaline earth metal compound throughout the mass of the reaction mixture is apparently achieved during the initial heating step, which is carried out at a relatively high temperature, at which this distribution of the alkaline earth metal compound is promoted. One of the initial reactions which is also believed to be essential to the process is, as mentioned above, the combination of the alkali metal oxide with the iron oxide present in the mixture to form an alkali metal ferrite, for example, potassium ferrite, KzO.Fe2O3: thus the product of the initial heating probably consists substantially of an alkali metal ferrite containing an alkaline earth metal compound uniformly distributed throughout its mass: it should be noted that the alkaline earth metal compound here referred to is not necessarily the same as that used in the initial reaction mixture. When this alkali metal ferrite is treated with water it is hydrolysed, with the liberation of an iron oxide which may be more or less hydrated and which has not itself been subjected to a high temperature: we have found that a so-called low temperature iron oxide, that is to say, an oxide which has not itself been subjected to a high temperature during its preparation, can be reduced at a lower temperature than can an oxide which has been subjected to a high temperature. Accordingly, the iron oxide obtained by the method of the invention can be reduced at a relatively low temperature, and this is advantageous since the use of a low temperature instead of a high temperature for the reduction is a further factor tending to minimise crystal growth of t e iron particles.

What I claim is:

1. A method of manufacturing a magnetisable powder which consists mainly of metallic iron and which is composed of particles of mean ultimate crystal size such that the greatest dimension of each crystal is not greater than 5,000 A., which method comprises the steps of heating a mixture of an iron compound of the group consisting of iron oxides, hydrated iron oxides and iron hydroxide, with alkaline earth metal compound of the group consisting of the alkaline earth metal oxides, hydroxides and carbonates, and an alkali metal compound of the group consisting of the alkali metal oxides, hydroxides and carbonates, the proportion of said alkaline earth metal compound being such that the final product contains from 0.1% to of alkaline earth metal by weight of the product, and the heating being carried out at a low temperature such that reaction takes place between said alkali metal compound and substantially all of said iron compound to form an alkali metal ferrite, and the alkaline earth metal compound is substantially uniformly distributed through the mixture, treating the product of said heating with water, whereby an aqueous solution is formed and an iron oxide containing solid residue is left, removing said aqueous solution and heating said solid residue at a temperature below 500 C. in a reducing atmosphere until the iron present in said residue is mainly reduced to the metallic state.

2. A method as claimed in claim 1 wherein said iron compound is alpha-ferric oxide of small crystal size and low density in the uncompressed state.

3. A method as claimed in claim 1 wherein the alkaline earth metal in the said alkaline earth metal comture is heated in an atmosphere of oxyge' p e -t1 is" a nifhe' ef tii grasp" eensistin'g'bt calcium, 's't'rohtiumandbariuin. I I t p, 4'. niethod as claimed ificlaith 1 V line metalin said alkali metal compound is member from the group consisti g of sodiiiin aiid etassiiini. I

, 5. A methodas claimedin claim l, wher"ein said iiiixes o 6, method as claimed in claim 1 wher i i Said solid residue is heated at a temperature between 250 C. and 500 C. in a stream of hydrogen flowing at a rate of at least 300 litres per hour for every 10 to 15 grams of solid residue until the iron present in said residue is mainly reduced to the metallic state.

7. A method of manufacturing a magnetisable powder which consists mainly of metallic iron and which is composed of particles of mean ultimate crystal size such that the greatest dimension of each crystal is not greater than 5,000 A., which method comprises the steps of heating a mixture of an iron compound of the group consisting of iron oxides, hydrated iron oxides and iron hydroxide with alkaline earth metal compound of the group consisting of the alkaline earth metal oxides, hydroxides and carbonates, and an alkali metal compound of the group consisting of the alkali metal oxides, hydroxides and carbonates, the proportion of said alkaline earth metal compound being such that the final product contains from 0.1% to 10% of alkaline earth metal by weight of the product, and the heating being carried out at a temperature in the range of 500800 C. such that reaction takes place between said alkali metal compound and substantially all of said iron compound to form an alkali metal ferrite, and the alkaline earth metal compound is substantially uniformly distributed through the mixture, treating the product of said heating with water, whereby an aqueous solution is formed and an iron oxide containing solid residue is left, removing said aqueous solution and heating said solid residue at a temperature below 500 C. in a reducing atmosphere until the iron present in said residue is mainly reduced to the metallic state.

8. A method of manufacturing a magnetisable powder which consists mainly of metallic iron and which is composed of particles of mean ultimate crystal size such that the greatest dimension of each crystal is not greater than 5,000 A., which method comprises the steps of heating a mixture of an iron compound of the group consisting of iron oxides, hydrated iron oxides and iron hydroxide, with an alkaline earth metal compound of the group consisting of the alkaline earth metal oxides, hydroxides and carbonates, and an alkali metal hydroxide, at a temperature in the range of 500 C. to 800 C., whereby the mixture is kept in a semi-liquid state during the treating due to fusion of said alkali metal hydroxide, and reaction takes place between said iron compound and said alkali metal hydroxide to form an alkali metal ferrite, the proportion of said alkaline earth metal compound being such that the final product contains from 0.1% to 10% of alkaline earth metal by weight of the powder, treating the product of said heating with Water, whereby an aqueous solution is formed and an iron oxide containing, solid residue is left, removing said aqueous solution, and heating said solid residue at a temperature below 500 C. in a reducing atmosphere until the iron present in said residue is mainly reduced to the metallic state.

9. A method of manufacturing a m agnetisable powder which consists mainly of metallic iron and which is composed of particles of mean ultimate crystal size such that the greatest dimension of each crystal is not greater than 5,000 A., which method comprises the steps of heating a mixture of an iron compound of the group consisting of iron oxides, hydrated iron oxides and iron hydroxide, with an alkaline earth metal compound of the group consisting of the alkaline earth metal oxides, hydroxides and carbonates, and an alkali metal carbonate, at a temperature in the range of 700 C. to 800 C; to produce a sintered mass, whereby reaction takes place between said iron compound and said alkali metal compound to form an alkali metal ferrite, the proportion of said alkaline earth metal compound being such'that the final product contains from 0.1% to 10% of alkaline earth metal by weight of the powder, treating said sintered mass with water, whereby an aqueous solution is formed and an iron oxide containing, solid residue is left, removing said aqueous solution, and heating said residue at a temperature state.

References Cited in the file of this patent UNITED STATES PATENTS Brill et a1 Nov. 14, 1939 Neel Feb. 14, 1950 

1. A METHOD OF MANUFACTURING A MAGNETISABLE POWDER WHICH CONSISTS MAINLY OF METALLIC IRON AND WHICH IS COMPOSES OF PARTICLES OF MEAN ULTIMATE CRYSTAL SIZE SUCH THAT THE GREATEST DIMENSION OF EACH CRYSTAL IS NOT GREATER THAN 5,000 A., WHICH METHOD COMPRISES THE STEPS OF HEATING A MIXTURE OF AN IRON COMPOUND OF THE GROUP CONSISTING OF IRON OXIDES, HYDRATED IRON OXIDES AND IRON IRON HYDROXIDE, WITH AN ALKALINE EARTH METAL COMPOUND OF THE GROUP CONSISTING OF THE ALKALINE EARTH METAL OXIDES, HYDROXIDES AND CARBONATES, AND AN ALKALI METAL COMPOUND OF THE GROUP CONSISTING OF THE ALKALI METAL OXIDES, HYDROXIDES AND CARBONATES, THE PROPORTION OF SAID ALKALINE EARTH METAL COMPOUND BEING SUCH THAT THE FINAL PRODUCT CONTAINS FROM 0.1% TO 10% OF ALAKALINE EARTH METAL BY WEIGHT OF THE PRODUCT,AND THE HEATING BEING CARRIED OUT AT A LOW TEMPERATURE SUCH THAT REACTION TAKES PLACE BETWEEN SAID ALKALI METAL COMPOUND AND SUBSTANTIALLY ALL OF SAID IRON COMPOUND TO FORM AN ALKALI METAL FERRITE, AND THE ALKALINE EARTH METAL COMPOUND IS SUBSTANTIALLY UNIFORMLY DISTRIBUTED THROUGH THE MIXTURE, TREATING THE PRODUCT OF SAID HEATING WITH WATER, WHEREBY AN AQUEOUS SOLUTION IS FORMED AND AN IRON OXIDE CONTAINING SOLID RESIDUE IS LEFT, REMOVING SAID AQUEOUS SOLUTION AND HEATING SAID SOLID RESIDUE AT A TEMPERATURE BELOW 500*C. IN A REDUCING ATMOSPHERE UNTIL THE IRON PRESENT IN SAID RESIDUE IS MAINLY REDUCED TO THE METALLIC STATE. 